Year	Percent
1970	41.09449
1971	37.65582
1972	37.41656
1973	40.09389
1974	39.30086
1975	34.51922
1976	33.53626
1977	32.49219
1978	32.55272
1979	34.22227
1980	33.95776
1981	32.75943
1982	31.59663
1983	30.12304
1984	30.06065
1985	30.01169
1986	29.9707
1987	30.94322
1988	33.04843
1989	33.86056
1990	34.51055
1991	34.22499
1992	32.48937
1993	30.67904
1994	29.54486
1995	29.88286
1996	30.86515
1997	29.951
1998	28.51517
1999	27.11881
2000	27.30713
2001	26.56051
2002	24.6555
2003	24.39723
2004	24.34862
2005	24.74947
2006	24.74863
2007	24.48345
2008	24.54785
2009	21.32367
2010	21.29749
2011	22.1033
2012	22.65442
2013	23.19091
2014	23.9161
2015	24.0053
2016	23.56428